Nonlinear optical loop mirrors (NOLMs), which are fiber versions of Sagnac interferometers, can perform many different functions in optical systems including multiplexing and demultiplexing, switching, amplifying, logical operations, pulse shaping, filtering, and signal regeneration. Processing speeds are extremely fast, accommodating bit rates approaching terabit per second speeds.
Pulse routing through nonlinear optical loop mirrors is controlled by the mechanism of interference. A two-by-two directional coupler divides a data (signal) pulse train into two pulse trains that counter propagate around a common loop of fiber. Phase modulation induced by intensities within the nonlinear optical regime of the fiber (the Kerr effect) alters the relative phases of the paired counter propagating pulses. Upon return to the directional coupler, the recombined pulses are switched between the input and output of the coupler in accordance with their interference properties. Constructively interfering pulses reflect back through the coupler's input, and destructively interfering pulses transmit through the coupler's output.
Normally, the directional coupler is a 3 dB coupler, splitting input pulse intensities equally between the counter propagating directions. If the optical properties exhibited by the fiber loop are symmetric in both directions of propagation, the returning pulses interfere constructively and reflect back through the coupler input. Asymmetries resulting in a ".pi." phase shift cause the returning pulses to interfere destructively and to transmit through the coupler output. Other phase shifts divide the intensities of individual pulses between the coupler input and output in accordance with their relative amounts of constructive and destructive interference.
The asymmetries can be arranged to affect all data pulses equally or to have a differential effect on selected data pulses such as on data pulses with certain characteristics, on particular patterns of data pulses, or even on individual data pulses. For example, my recently issued U.S. Pat. No. 5,655,039, which is hereby incorporated by reference, constructs the loop of a non-linear optical loop mirror with dispersion-tapered fiber to produce a differential effect on data pulses having different widths or intensities. Individual data pulses or patterns of data pulses can be differentially affected by using specially timed control (clock) pulses, which limit asymmetric effects to periods of overlap with selected data pulses.
In a preferred mode of operation, the control pulse starts just ahead or just behind the expected position of the selected data pulse and ends in the opposite position. Between the two positions, the control pulse overlaps the selected data pulse for a sufficient duration and with a sufficient intensity to produce a .pi. phase shift along the entire selected data pulse. The phase shift is induced by temporary changes in the effective refractive index of the fiber loop in the presence of the control pulse, which has an intensity within the nonlinear optical regime of the fiber.
However, a variety of differential effects on data pulses causes them to drift from their expected positions--a phenomenon referred to as "timing jitter". Any delay between the expected arrivals of the control and selected data pulses reduces the transmission efficiency of the selected data pulse because less of the selected data pulse undergoes the required .pi. phase shift. The effects of timing jitter on transmission efficiency can be reduced by starting the control pulse farther behind the selected data pulse and ending the control pulse farther ahead of the selected data pulse (i.e., increase the so-called "walkoff distance"). However, the increased walkoff distance (as it approaches the bit period) can also contribute to increased crosstalk caused by the unintended transmission of adjacent pulses.
A convenient measure of how well timing jitter can be accommodated is the so-called "switching window", having a width defined as the full width at half maximum (FWHM) of a transmission efficiency curve plotted as a function of the relative delay between the control and selected data pulses. The optimum window width balances the need to accommodate timing jitter (i.e., minimize intensity variations between transmitted data pulses) with the need to avoid crosstalk (i.e., minimize transmission of unselected data pulses). The shape of the switching window can also be optimized to further these objectives. The top of the switching window is preferably flattened to reduce intensity variations within a limited range of timing jitter, and the sides are preferably steepened (i.e., approach a more nearly vertical slope) to reduce crosstalk. The ideal shape of a switching window is described by a rectangle function.